The conducting airways of humans are lined by a superficial layer of epithelial cells which comprise an important primary line of defense to the entire respiratory tract. This superficial cellular layer consists primarily of mucus-producing (goblet) cells and ciliated cells. These cells function in a coordinated fashion to entrap inhaled biological and inert particulates and remove them from the airways. While this “mucociliary escalator” functions with great efficiency in the face of potentially injurious stimuli, it is a delicately balanced system relying on maintenance of appropriate complements of ciliated and mucus-producing cells and the normal functioning of those cells to accomplish effective clearance. Perturbations in epithelial cell type distribution and function can lead to adverse health effects.
Ciliated cells represent approximately 80% of the epithelial cells residing on luminal borders of the large airways. While they are the most prevalent epithelial cell type lining the airways, many studies suggest that they also are among the most vulnerable to injury by infection, irritant, and pollutant exposure. The identifying characteristic of ciliated cells, are the highly organized appendages of the cell, i.e., the cilia which cover the luminal border.
Mucus and other non-ciliated cells represent approximately 20% of the epithelial cells lining the luminal borders of the large airways. Mucus cells often are distended with secretory product and exhibit a characteristic “goblet” shape. Together with the submucosal glands, goblet cells secrete high molecular weight mucus glycoproteins (mucins). Goblet cells are thought to have the potential to produce markedly more mucus than do the glands, especially in response to injury such as environmental pollutants and other noxious elements such as tobacco/cigarette smoke.
Other non-ciliated cells with fewer or no granules also may be present along the luminal border. These may represent mucus cells which have emptied their contents onto the luminal surface or cells which have not yet differentiated. The entire epithelial layer sits on a basement lamina comprised of collagen and connective tissue. All the cells of the epithelial layer are anchored to this “basement membrane.”
Chronic bronchitis is a non-infectious inflammatory disease typically resulting from airway injury due to a noxious element (usually smoking). It is defined by cough with productive sputum of three months duration for two consecutive years. It is further characterized by excess mucus (mucus hyperactivity/hypersecretion/hyperplasia of goblet cells) in the bronchi, damage to cilia and loss of ciliated cells. Noxious stimuli lead to airway inflammation with swelling of the lamina propria leading to thickening of the airway wall, and this functional narrowing causes shortness of breath. More specifically, this injury causes over-proliferating goblet cells to over-produce a thick viscous, acidic mucus which is difficult to clear due to cilia dysfunction. The acidic mucous in chronic bronchitis leads to inflammation of the airway wall and varies in viscosity.
Asthma is a chronic respiratory disease characterized by bronchial inflammation, increased airway smooth muscle and airway hyper-responsiveness, in which airways narrow (constrict) excessively or too easily in response to a stimulus. Asthma episodes or attacks cause narrowing/constriction of the airways, which makes breathing difficult. Asthma attacks may occur at irregular intervals and be triggered by allergens or irritants that are inhaled into the lungs or by stress, cold air, viral infections or other stimuli. Asthma is sometimes, but not always, associated with mucus hyperactivity.
Airway hypersecretion is a feature of other airway diseases as well, including chronic obstructive pulmonary disease (COPD), cystic fibrosis, viral bronchitis, and bronchiolitis.
In an individual suffering from hypersecretion, mucus accumulates in the airways and may cause airway obstruction. Airway submucosal glands and goblet cells lining the airway epithelium secrete mucus, an adhesive, viscoelastic gel composed of water, carbohydrates, proteins, and lipids. In a healthy individual, mucus is a primary defense against inhaled foreign particles and infectious agents and is cleared by active columnated cilial cells/movement which assists in clearing the mucus in an upward direction where it is either swallowed or eliminated via a productive cough. Mucus traps these particles and agents and facilitates their clearance while also preventing tissues from drying out. Small airways that contain goblet cells as well as peripheral airways and which cannot be cleared by cough are particularly vulnerable to mucus accumulation and gradual obstruction by mucus.
Conventional treatments for individuals suffering from airway hypersecretion or chronic bronchitis include use of systemic or inhaled corticosteroids, anticholinergics, antibiotic therapy, bronchodilators (e.g., methylxanthines), short or long-acting beta2-agonists which relax the muscles in the airways to relieve symptoms, aerosol delivery of “mucolytic” agents (e.g., water, hypertonic saline solution), and oral administration of expectorants (e.g., guaifenesin). It should be noted that while these medications are variably approved by the FDA for use in COPD they are not specific for chronic bronchitis with the exception of roflumilast, an inhibitor of an enzyme called phosphodiesterase type 4 (PDE-4).
Many of the above described medications have serious side effects. For example, inhaled corticosteroids can cause thrush (a yeast infection of the mouth), cough, or hoarseness, and systemic corticosteroids have even more severe side effects, such as delayed sexual development, changes in menstrual cycle, weight gain, and increased blood sugar (diabetes). The side effects of methylxanthines include severe nausea, tremors, muscle twitching, seizures, and irregular heartbeat. Roflumilast commonly induces significant diarrhea. Patient compliance is often low due to these side-effects.
Interventional approaches to managing occluded airways include surgery, mechanical debulking, brachytherapy, stents, photodynamic therapy, and thermal modalities, such as electrocautery, laser, argon plasma coagulation, and bronchial thermoplasty. Bronchial thermoplasty is a procedure designed to help control severe asthma by reducing the mass of airway smooth muscle by delivering thermal energy to the airway wall, heating the tissue in a controlled manner. Bronchial thermoplasty with RF energy creates a deep ablation effect down to the level of the airway smooth muscle creating a reparative healing that results in scar tissue which is fibrotic in nature. Hyper-thermal treatment denatures proteins, and causes enyzme inactivation and prevents collagen remodeling. Accordingly, bronchial thermoplasty patients cannot be re-treated in the same areas. Cryoprobes have also been used in airway management, but their use can be tedious and time-consuming because of surface area limitations of the probes, which requires contact between the probe and the surface of the targeted lesion or tissue.
Reports of promising results from use of low-pressure spray cryotherapy for ablation of esophageal lesions (Barrett esophagus, dysplasia and esophageal cancers) led Krimsky, et al. to gauge the safety of using cryospray in airway tissues. Krimsky, et al., 2009. Krimsky, et al., reported performing spray cryotherapy on 21 subjects who were scheduled for lung resection for treatment of lung cancer, carcinoid tumor and mycobacterial infection. Treatment areas were directed to normal and unrestricted portions of the airway distal to planned anastomotic sites. All sites received a targeted delivery of low-pressure (2-3 psi) liquid nitrogen of identical dosimetry, 2 cycles of 5-second spray with a 60-second interval thaw. All patients had treatment times shorter than 5 minutes. Post-treatment bronchoscopic and histologic examinations of airways were conducted from less than 1 day to 106 days after treatment.
Findings from the treated areas revealed varying levels of cryonecrosis, limited to the mucosal and submucosal layers (approximately 1.5 mm), and changes consistent with recent tissue injury with no damage to connective tissue. Krimsky, et al. reported loss of epithelium and airway smooth muscle, edema, and damaged submucosal glands at early time points post-treatment, followed by adjacent re-epithelialization and healing centrally from the margin of the injury. Complete re-epithelialization of the airway mucosa and a thinned or absent smooth muscle layer, as well as some continued thinning of the submucosal glands was reported to persist to 106 days after treatment.
Krimsky, et al. reported that these initial safety and histologic assessments suggested that spray cryotherapy may be safe and conducive to treatment of the airways by causing focal injury to the cellular elements of treated tissue without damage to underlying connective tissue, i.e., the extracellular matrix. Acknowledging the small number of the subjects in the study, and particularly noting that only normal, unobstructed airways were treated, Krimsky et al. nevertheless posited that the results of that study suggest treatment possibilities in human thoracic diseases.
Notably, in addition to only treating healthy unobstructed tissue (no treatment of regions characterized by excess goblet cells, hypersecretion, or damaged or lost cilia), Krimsky, et al. reported no observations concerning mucous production, goblet cell population or proliferation, and or cilia/ciliated cell population, either pre- or post-treatment. Additionally, Krimsky et al. made no observations or suggestions that cryospray treatment can actually cause change in architecture of diseased/damaged tissue, and no suggestion that diseased sections could be regenerated as healthy tissue. Moreover, there have been no published studies since Krimsky, et al. that have addressed these questions. Indeed, as of this writing, there are no medications or devices today that propose reduction in mucous secreting cells, and/or remodeling of cilia.